1. Field of the Invention
The present invention relates to a pulley, and particularly to a pulley which is mounted on the outer end of a crankshaft of an engine of a vehicle or the like.
2. Description of the Related Art
Conventionally, a pulley which is secured to a crankshaft is provided with a rubber damper to reduce such a phenomenon as torsional and flexural resonance of the crankshaft as disclosed in Japanese Laid-open Patent Application No. Sho-59-40060 and Japanese Laid-open Utility Model Application No. Hei-2-80252.
Since the phenomenon such as the torsional and flexural resonance of the crankshaft as described above occurs particularly at a high rotational number area (for example, above 2000 rpm) of the crankshaft, the characteristics of the rubber damper are designed so that the suppression of the above phenomenon is most effectively performed at this rotational number area.
It has been generally known that the crankshaft is advanced or delayed due to unevenness of explosion of a combustion chamber, resulting in variation of the rotational number. The variation of the rotational number of the crankshaft is more remarkable particularly at a low rotational number area such as an idling area (for example, around 1000 rpm). The variation of the rotational number of the crankshaft directly causes the rotational variation of the pulley, and thus the tension of a belt which is suspended around the pulley varies, so that the belt is slipped on the pulley to induce a belt noise or wear on the belt.
When the characteristics of the rubber damper is designed so that the variation of the rotational number of the crankshaft particularly at the idling rotation area is reduced by the rubber damper as described above, not only the suppression effect of the resonance phenomenon which is the original purpose to use the rubber damper is lowered, but also the durability of the rubber damper itself is reduced.